Implantable medical devices are commonly used to treat patients with various conditions of the heart including life threatening tachycardia and various types of bradycardia. As an example of such devices, implantable cardiac rhythm management (CRM) systems generally include pulse generators and stimulation leads, and have electrodes implanted in or about the heart. Such devices typically function by administering pulses of electrical energy to the heart through the electrodes.
Many early CRM devices included a single electrode on the stimulation lead and electrical stimulation was delivered by passing a current from the pulse generator, through the stimulation lead, through the single electrode on the stimulation lead, into the patient's cardiac tissue, and back to the housing of the pulse generator to complete the circuit. Thus, the electrode on the stimulation lead served as one pole and the housing of the pulse generator itself served as the second pole. This type configuration, where the housing of the pulse generator serves as one pole, is generally known as a unipolar configuration.
Systems were also developed that included two electrodes on the stimulation lead. This allowed for the administration of electrical stimulation with one electrode of the stimulation lead serving as the first pole and the other electrode of the stimulation lead serving as the second pole. This type of configuration is known as a bipolar configuration.
Over time, stimulation leads have been developed having more than two electrodes. For example, some stimulation leads designed for use with CRM systems can include a first shocking coil in addition to a second shocking coil, along with a tip electrode and ring electrode, thus having four electrodes in total. In addition, stimulation leads designed for use with neurological stimulation systems have included four or more electrodes.
However, delivering a pulse of electrical stimulation only requires the use of two poles. As such, with many modern stimulation leads many different combinations of electrodes can be used in order administer electrical stimulation. The particular combination of electrodes used to deliver electrical stimulation can also be referred to as the “vector” because those electrodes determine the starting point and the ending point for electrical current moving through the patient's tissue. Generally, the clinician must choose which vector should be used by the device for purposes of sensing, pacing, and/or shocking. The clinician can input their choice into an external programmer device, such as a programmer/recorder/monitor device, which can in turn interface with implanted device and program it to use the desired vector.